Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap, is usually coupled downstream of the three way catalytic converter. The LNT stores exhaust gas components such as oxidants, i.e., NOx and oxygen, when the engine is operating at a lean air-fuel ratio, and releases and reduces (purges) them when the engine is operating at a rich or stoic air-fuel ratio.
Over time, the ability of the LNT to store exhaust gas components can decrease due to such factors as sulfur deposits (SOx) from the fuel. Therefore, when the LNT storage capacity is sufficiently reduced, a SOx purge has to be performed. Since SOx purges result in fuel economy penalties, it is desirable not to purge unnecessarily. Thus, in order to maintain adherence to emission standards and obtain fuel economy benefits of a lean burning engine, the capacity of the LNT to store exhaust gas components needs to be monitored. The LNT efficiency can be inferred from the amount of oxygen that the LNT can store. One such method and system are described in U.S. Pat. No. 5,743,084. The system includes an LNT and an upstream and downstream oxygen sensors coupled to the LNT. The method correlates the change in the oxygen storage capacity to the change in the amount of time for the downstream oxygen sensor to switch to rich once a purge of the LNT is initiated. During rich operation, the fuel in the rich exhaust mixture entering the LNT will react with the oxygen stored in the LNT and therefore the tailpipe sensor will not switch to reading rich until all of the stored oxygen is depleted. The decrease in the lean to rich switch time of the downstream sensor indicates the overall decrease in the oxygen storage capacity of the LNT.
The inventors herein have recognized a disadvantage with this approach. Namely, when capacity estimates are performed under normal or high load driving conditions, i.e., at high space velocity, the reductant present in the exhaust gas entering the LNT does not have enough time to react with the stored oxygen. Therefore, not all of the oxygen is purged during high load operation resulting in an inaccurate efficiency estimate. Further, reductant wastage occurs due to the fact that it blows through the LNT at high speed.